1. The Field of the Invention
The invention relates to well bore tools and, more particularly, to well bore tools having improved outer surface configurations and performance characteristics for improved efficiency in borehole drilling.
2. History of the Prior Art
Borehole wall engaging tools, such as stabilizers, reamers and keyseat wipers, are frequently used in sections of the pipe or drill string of oil and gas well drilling rigs. Stabilizers are essentially enlarged sections of the drill string which provide limited surface area radially outwardly of the drill pipe and which tend to center said drill string and the drill bit within the borehole. Reamers also serve to center the drill bit within the borehole and, additionally, to cut and shape the wall of said borehole behind said bit. Keyseat wipers are used to smooth out the borehole wall by removing small heterogenities and other deviations from said borehole wall as the drill bit continues to move downwardly through varying earth formations.
More specifically, drill bits for drilling through earth and rock are generally designed to rotate about their own centers. Stabilizing the drill bit centrally within the borehole ensures the maximum specified drill penetration rate and maximum downhole control. Uncontrolled or unstabilized drill pipe lengths or sections of the drill string manifest a "flopping" action within the borehole and become subject to torsional vibrations during operation. These conditions contribute to power loss as well as equipment breakdown due to frictional wall engagement from the drill string which decreases the life of the pipe lengths and the tool joints therebetween.
In order to maintain the needed downhole concentricity, the drill bit is preferably followed closely by a reamer to centrally hold said bit within the borehole. The drill string generally includes periodically spaced stabilizers to hold its upper and intermediate sections concentric within the borehole. Without stabilization, the drill pipe tends to move out of concentricity producing undesirable chatter and deleterious wear. The drill string may also bump along the borehole wall thereby increasing power loss and wear on said drill string.
In the event that borehole wall deviations do occur, such as keyseats, it is preferable that they be removed and smoothed over as early as possible to prevent the hanging up of any portion of the drill string during subsequent insertion and/or removal from the borehole. A keyseat wiper in a borehole wall engaging tool having a slight downward taper which functions upon rotation of said tool as well as axial movement to smooth away pits or other deviations in the borehole wall.
The prior art has addressed the design of such borehole wall engaging tools by providing downhole tools with either radially outwardly extending ribs or individual spaced apart protrusions which selectively engage said borehole wall. Ribbed stabilizers are of generally two types, having either longitudinally extending plates or ribs spiraled about the exterior or outer surface of a stabilizer. Initially, rib-type stabilizers are made to conform to the radius of the drill bit and their external surfaces engage and rub against the borehole wall in order to maintain concentricity of the drill string with said borehole. This is also generally true of other prior art borehole wall engaging tools, such as reamers and wipers.
The outer exposed surfaces of these stabilizers and reamers are designed to scrape and rub against the borehole wall during downhole operation. Because the borehole generally extends through a multitude of various earth and rock formations, there is considerable abrasion between the tool and borehole which quickly wears the outer surfaces of such ribs or plate elements. This abrasion quickly ruins the accuracy of the radial dimensions of the outer surfaces of the tool. It may also be seen that the borehole wall engaging tools, which have longitudinally and circumferentially spaced discrete protrusions of the prior art variety, are then subject to the type of wear which results in a lack of concentricity within the borehole. This, in turn, results in a lack of longitudinal-transverse stability (i.e. directional control) of the tool within the borehole and a myriad of related operational problems.